32 research outputs found
Quantum and Classical Noise in Practical Quantum Cryptography Systems based on polarization-entangled photons
Quantum-cryptography key distribution (QCKD) experiments have been recently
reported using polarization-entangled photons. However, in any practical
realization, quantum systems suffer from either unwanted or induced
interactions with the environment and the quantum measurement system, showing
up as quantum and, ultimately, statistical noise. In this paper, we investigate
how ideal polarization entanglement in spontaneous parametric downconversion
(SPDC) suffers quantum noise in its practical implementation as a secure
quantum system, yielding errors in the transmitted bit sequence. Because all
SPDC-based QCKD schemes rely on the measurement of coincidence to assert the
bit transmission between the two parties, we bundle up the overall quantum and
statistical noise in an exhaustive model to calculate the accidental
coincidences. This model predicts the quantum-bit error rate and the sifted key
and allows comparisons between different security criteria of the hitherto
proposed QCKD protocols, resulting in an objective assessment of performances
and advantages of different systems.Comment: Rev Tex Style, 2 columns, 7 figures, (a modified version will appear
on PRA
Tailoring Single and Multiphoton Probabilities of a Single Photon On-Demand Source
As typically implemented, single photon sources cannot be made to produce
single photons with high probability, while simultaneously suppressing the
probability of yielding two or more photons. Because of this, single photon
sources cannot really produce single photons on demand. We describe a
multiplexed system that allows the probabilities of producing one and more
photons to be adjusted independently, enabling a much better approximation of a
source of single photons on demand.Comment: 4 pages, LaTex, 2 figures, twocolumn and RevTex Style for PR
A high-flux source of polarization-entangled photons from a periodically-poled KTP parametric downconverter
We have demonstrated a high-flux source of polarization-entangled photons
using a type-II phase-matched periodically-poled KTP parametric downconverter
in a collinearly propagating configuration. We have observed quantum
interference between the single-beam downconverted photons with a visibility of
99% and a measured coincidence flux of 300/s/mW of pump. The
Clauser-Horne-Shimony-Holt version of Bell's inequality was violated with a
value of 2.711 +/- 0.017.Comment: 7 pages submitted to Physical Review
Reducing the communication complexity with quantum entanglement
We propose a probabilistic two-party communication complexity scenario with a
prior nonmaximally entangled state, which results in less communication than
that is required with only classical random correlations. A simple all-optical
implementation of this protocol is presented and demonstrates our conclusion.Comment: 4 Pages, 2 Figure
An arbitrated quantum signature scheme
The general principle for a quantum signature scheme is proposed and
investigated based on ideas from classical signature schemes and quantum
cryptography. The suggested algorithm is implemented by a symmetrical quantum
key cryptosystem and Greenberger-Horne-Zeilinger (GHZ) triplet states and
relies on the availability of an arbitrator. We can guarantee the unconditional
security of the algorithm, mostly due to the correlation of the GHZ triplet
states and the use of quantum one-time pads.Comment: 10 pages, no figures. Phys. Rev. A 65, (In press
Experimental preparation of Werner state via spontaneous parametric down-conversion
We present an experiment of preparing Werner state via spontaneous parametric
down-conversion and controlled decoherence of photons in this paper. In this
experiment two independent BBO (beta-barium borate) crystals are used to
produce down-conversion light beams, which are mixed to prepare Werner state.Comment: 6 pages, 4 figures and 2 table
Maximizing the entanglement of two mixed qubits
Two-qubit states occupy a large and relatively unexplored Hilbert space. Such
states can be succinctly characterized by their degree of entanglement and
purity. In this letter we investigate entangled mixed states and present a
class of states that have the maximum amount of entanglement for a given linear
entropy.Comment: 4 pages, 3 figure
Entanglement Creation Using Quantum Interrogation
We present some applications of high efficiency quantum interrogation
("interaction free measurement") for the creation of entangled states of
separate atoms and of separate photons. The quantum interrogation of a quantum
object in a superposition of object-in and object-out leaves the object and
probe in an entangled state. The probe can then be further entangled with other
objects in subsequent quantum interrogations. By then projecting out those
cases were the probe is left in a particular final state, the quantum objects
can themselves be left in various entangled states. In this way we show how to
generate two-, three-, and higher qubit entanglement between atoms and between
photons. The effect of finite efficiency for the quantum interrogation is
delineated for the various schemes.Comment: 7 pages, 13 figures, Submitted to PR
Quantum entanglement with acousto-optic modulators: 2-photon beatings and Bell experiments with moving beamsplitters
We present an experiment testing quantum correlations with frequency shifted
photons. We test Bell inequality with 2-photon interferometry where we replace
the beamsplitters by acousto-optic modulators, which are equivalent to moving
beamsplitters. We measure the 2-photon beatings induced by the frequency
shifts, and we propose a cryptographic scheme in relation. Finally, setting the
experiment in a relativistic configuration, we demonstrate that the quantum
correlations are not only independent of the distance but also of the time
ordering between the two single-photon measurements.Comment: 14 pages, 16 figure
Suppression of decoherence in quantum registers by entanglement with a nonequilibrium environment
It is shown that a nonequilibrium environment can be instrumental in
suppressing decoherence between distinct decoherence free subspaces in quantum
registers. The effect is found in the framework of exact coherent-product
solutions for model registers decohering in a bath of degenerate harmonic
modes, through couplings linear in bath coordinates. These solutions represent
a natural nonequilibrium extension of the standard solution for a decoupled
initial register state and a thermal environment. Under appropriate conditions,
the corresponding reduced register distribution can propagate in an unperturbed
manner, even in the presence of entanglement between states belonging to
distinct decoherence free subspaces, and despite persistent bath entanglement.
As a byproduct, we also obtain a refined picture of coherence dynamics under
bang-bang decoherence control. In particular, it is shown that each
radio-frequency pulse in a typical bang-bang cycle induces a revival of
coherence, and that these revivals are exploited in a natural way by the
time-symmetrized version of the bang-bang protocol.Comment: RevTex3, 26 pgs., 2 figs.. This seriously expanded version accepted
by Phys.Rev.A. No fundamentally new content, but rewritten introduction to
problem, self-contained introduction of thermal coherent-product states in
standard operator formalism, examples of zero-temperature decoherence free
Davydov states. Also fixed a typo that propagated into an interpretational
blunder in old Sec.3 [fortunately of no consequence